Quadraphase modulator/circuits for digital communication systems are known. For example, the HPMX-2001 is a silicon monolithic quadraphase shift keyed modulator with a typical local oscillator (LO) operating frequency range of DC-2000 MHz and typical I/Q bandwidth of DC-700 MHz. Such devices are useful for, among other things, digital cellular radio, RF data links, vector generators, AM modulators, and single sideband mixers. Typical I/Q modulator performance of this and similar chips is limited by the quadrature quality of the LO signals. Quadrature phase errors occur due to process variations, circuit component mismatch and circuit parasitics. In addition, it is difficult to make accurate broad band quadrature phase shifters. Phase errors in the I and Q LO signals in complex vector modulators, for example, limit sideband suppression critical to meeting system specifications for ADC (IS-54) and GSM digital cellular phone transmitters.